A "cohesive adhesive" or "cold seal" is an adhesive which is nontacky to the touch, yet adheres to itself with pressure. Cold seals are employed for a variety of uses, particularly in various packaging applications such as for food (i.e. flexible packaging for candy wrappers, chips etc.), sterilizable medical packaging, self-seal and tamper evident envelopes, banding for paper money, napkins, and clothing; and protective packaging such as fold over "blister" packages for hardware and small parts. Additionally, cold seal adhesives have found utility as anti-skid coatings and for release-paper free tapes such as tabless diaper tapes.
Natural latex based products have dominated the cold seal market for quite some time, since natural latex is inherently cohesive. Natural latex based cold seals have several deficiencies. For example, in the medical packaging area, natural latex has been identified as a skin sensitizer due to it containing certain proteins. Natural latex is also susceptible to aging effects such as yellowing and changes in the bond strength because of oxidation. Another deficiency of natural latex relates to the effect of irradiation, which is required for the sterilization of packaged medical bandages and devices. Irradiation causes the natural latex to crosslink, significantly changing the bond properties.
Certain hot melt adhesives have also found utility as cold seals. EP 531618 published Mar. 17, 1993 teaches cohesive cold seal compositions which include about 60 to about 95 parts by weight of an amorphous polyalphaolefin (APAO) polymer and about 1 to 30 parts of a crystalline aliphatic hydrocarbon wax. Exemplified are compositions based on REXTAC and VESTOPLAST APAO derived from Ziegaler-Natta catalysis. Such polyalphaolefins characteristically have a relatively broad polydispersity (Mw/Mn). Further, Rextac 2280 and Rextac 2315 are ethylene-propylene copolymers containing relatively low concentrations of ethylene. These APAO based compositions are compounded and disadvantageously exhibit poor self seal properties and low peel strengths.
Homogeneous linear and substantially linear ethylene polymers are prepared using single-site or metal locene catalysts. Homogeneous ethylene/.alpha.-olefin interpolymers differ from APAO, with regard to homogeneity, molecular weight distribution (M.sub.w /M.sub.n), as well as comonomer (.alpha.-olefin) content.
Tse et al., U.S. Pat. No. 5,530,054, claims a hot melt adhesive composition consisting essentially of: (a) 30-70 weight percent of a copolymer of ethylene and about 6 to about 30 ) weight percent of a C.sub.4 to C.sub.20 .alpha.-olefin produced in the presence of a catalyst composition comprising a metallocene and an alumoxane and having an M.sub.w of from about 20,000 to about 100,000; and (b) a hydrocarbon tackifier which is selected from a recited list. Exemplified are compositions consisting of 45 weight percent of ethylene/butene-1 copolymer having a specific gravity of either 0.898 g/cm.sup.3 or 0.901 g/cm.sup.3.
Tse et al, U.S. Pat. No. 5,548,014, claims a hot melt adhesive composition comprising a blend of ethylene/.alpha.-olefin copolymers wherein the first copolymer has a M.sub.w from about 20,000 to about 39,000 and the second copolymer has a M.sub.w from about 40,000 to about 100,000. Each of the hot melt adhesives exemplified comprises a blend of copolymers, contains 45 weight percent copolymer, with at least one of the copolymers having a polydispersity greater than 2.5. Furthermore, the lowest density copolymer exemplified has a specific gravity of 0.894 g/cm.sup.3.
Lakshmanan et al., U.S. Pat. No. 5,397,843, teaches blended polymer compositions comprising an admixture of a copolymer of ethylene and an .alpha.-olefin and an amorphous polypropylene and/or amorphous polyolefin, or mixtures thereof. The single ethylene and .alpha.-olefin exemplified, FLEXOMER 9042, is described as having a 1-butene content of 15 weight percent, a melt index of 5.0 g/10 minutes, a crystallinity level of 26 percent and a density of 0.900 g/cm.sup.3.
Hence, single-site catalyzed or metallocene polyolefins have found utility for use in hot melt adhesive compositions. However, the hot melt adhesives exemplified in Tse et al. and Lakshmanan et al. employ relatively high density metallocene polyolefins causing such compositions to be unsuitable for use in cohesive cold seals.
Industry would find advantage in cold seal compositions that overcome the disadvantages of the natural latex and amorphous polyalphaolefin based compositions.